Overview of Transcription

Transcription is a process in which one DNA strand is used as template to synthesize a complementary RNA.  The following is an example:

Note that uracil (U) of RNA is paired with adenine (A) of DNA.  There are a few different names for these nucleic acid strands.  The DNA strand which serves as the template may be called "template strand", "minus strand", or "antisense strand".  The other DNA strand may be termed "non-template strand", "coding strand", "plus strand", or "sense strand".  Since both DNA coding strand and RNA strand are complementary to the template strand, they have the same sequences except that T in the DNA coding strand is replaced by U in the RNA strand.

Figure 4-B-1.  Schematic illustration of transcription.  (a) DNA before transcription.  (b) During transcription, the DNA should unwind so that one of its strand can be used as template to synthesize a complementary RNA.

Growth of a nucleic acid strand is always in the 5' to 3' direction.  This is true not only for the synthesis of RNA during transcription, but also for the synthesis of DNA during replication.  The enzymes, called polymerases, are used to catalyze the synthesis of nucleic acid strands. RNA strands are synthesized by RNA polymerases.  DNA strands are synthesized by DNA polymerases.

The entire transcription process should involve the following essential steps:

(i) Binding of polymerases to the initiation site.  The DNA sequence which signals the initiation of transcription is called the promoter.  Prokaryotic polymerases can recognize the promoter and bind to it directly, but eukaryotic polymerases have to rely on other proteins called transcription factors.

(ii) Unwinding (melting) of the DNA double helix (Figure 4-B-1). The enzyme which can unwind the double helix is called helicase.  Prokaryotic polymerases have the helicase activity, but eukaryotic polymerases do not.  Unwinding of eukaryotic DNA is carried out by a specific transcription factor.

(iii) Synthesis of RNA based on the sequence of the DNA template strand.  RNA polymerases use nucleoside triphosphates (NTPs) to construct a RNA strand.

(iv) Termination of synthesis.  Prokaryotes and eukaryotes use different signals to terminate transcription.  [Note: the "stop" codon in the genetic code is a signal for the end of peptide synthesis, not the end of transcription.]

Transcription in eukaryotes is much more complicated than in prokaryotes, partly because eukaryotic DNA is associated with histones, which could hinder the access of polymerases to the promotor.